In one type of floor construction rectangular metal ducts and conduit are secured in place on a base concrete slab and are each provided with one or more individual fittings which are adapted to provide access thereto after the ducts have been covered with concrete to provide a working floor surface. In such construction the ducts and conduit are joined individually end to end by sleeve junctions to provide desired lengths out of standard lengths of duct and conduit, and are secured to the base slab by support brackets which maintain the ducts and conduit at appropriate elevations relative to the base slab.